关于 C++ 的 new、delete 与 C 的 malloc、free 的区别

因为平台实时库部分的封装有点小恶心，如果取数据，用户必须传入一个有效的buf的初始地址和长度，接口内部的实现会在长度不够的情况下自动realloc这块buf，所以用户的缓存必须是malloc出来的。部门的个别小伙儿喜欢用了new来分配内存，当然了buf的长度足够大，是不会出问题的，如果一开始分配的比较小，问题就来了。纠结于因为这个问题，老是会被问及C/C++内存管理的差异，特写了个小程序以验证（写的不是很严谨，仅为主题，分别在redhat 5.6 x86_64 + gcc-4.9.2，windows 7
x64 + gcc-4.9.2 编译运行通过）：

代码：

#include <iostream>
#include <typeinfo>
#include <stdlib.h>
#include <stdio.h>

void t_cpu()
{
#define print_type(t) std::cout << "Size of " << #t <<  ": " << sizeof(t) << std::endl
print_type(char);
print_type(unsigned char);
print_type(short);
print_type(unsigned short);
print_type(int);
print_type(unsigned int);
print_type(size_t);
print_type(long);
print_type(unsigned long);
print_type(long long);
print_type(unsigned long long);
print_type(float);
print_type(double);
print_type(long double);
print_type(void*);

}

class __T {
public:
int _1;
int _2;

__T(){
_1 = 1;
_2 = 2;
}
~__T(){
printf("~");
}

void init()
{
_1 = 7;
_2 = 8;
}

void test()
{
_1 += _2;
}
};

void t_new(size_t sl)
{
__T *pchar = new __T[sl];
size_t  *psize  = (size_t*)(((char*)pchar)-sizeof(size_t));
printf("\nTest new and delete \nsize: %ld pchar: %p psize: %p *psize: %ld \n", sl, pchar, psize, *psize);
delete []pchar;
printf("\n");
}

void t_malloc(size_t sl)
{
printf("\nTest malloc and free \nsize of class __T: %ld  \n", sizeof(class __T));
__T *pchar = (__T *)malloc(sl * sizeof(class __T));
size_t  *psize  = (size_t*)(((char*)pchar)-sizeof(size_t));
printf("size: %ld pchar: %p psize: %p *psize: %ld \n", sl, pchar, psize, *psize);
free(pchar);
}

void t_new_free(size_t sl)
{
__T *pchar = new __T[sl];
size_t  *psize  = (size_t*)(((char*)pchar)-sizeof(size_t));
printf("\nTest new and free \nsize: %ld pchar: %p psize: %p *psize: %ld \n", sl, pchar, psize, *psize);
//delete []pchar;
for (__T *t = pchar; t < pchar + sl; ++t) {
t->~__T();
}

free(psize);
printf("\n");
}

void t_malloc_delete(size_t sl)
{
printf("\nTest malloc and delete \nsize of class __T: %ld  \n", sizeof(class __T));
size_t *psize = (size_t *)malloc(sl * sizeof(class __T) + sizeof(size_t));
__T  *pchar  = (__T*)(((char*)psize)+sizeof(size_t));
*psize = sl;
//for (__T *t = pchar; t < pchar + sl; ++t) {
//    t->init();
//}
printf("size: %ld pchar: %p psize: %p *psize: %ld \n", sl, pchar, psize, *psize);
delete [] pchar;
printf("\n");
}

int main()
{
t_cpu();
t_new(48);
t_malloc(32);
t_new_free(16);
t_malloc_delete(8);
return 0;
}

输出（redhat）：

Size of char: 1
Size of unsigned char: 1
Size of short: 2
Size of unsigned short: 2
Size of int: 4
Size of unsigned int: 4
Size of size_t: 8
Size of long: 8
Size of unsigned long: 8
Size of long long: 8
Size of unsigned long long: 8
Size of float: 4
Size of double: 8
Size of long double: 16
Size of void*: 8

Test new and delete
size: 48 pchar: 0x2442018 psize: 0x2442010 *psize: 48
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Test malloc and free
size of class __T: 8
size: 32 pchar: 0x2442010 psize: 0x2442008 *psize: 273

Test new and free
size: 16 pchar: 0x2442018 psize: 0x2442010 *psize: 16
~~~~~~~~~~~~~~~~

Test malloc and delete
size of class __T: 8
size: 8 pchar: 0x2442018 psize: 0x2442010 *psize: 8
~~~~~~~~

输出（Windows）：

Size of char: 1
Size of unsigned char: 1
Size of short: 2
Size of unsigned short: 2
Size of int: 4
Size of unsigned int: 4
Size of size_t: 4
Size of long: 4
Size of unsigned long: 4
Size of long long: 8
Size of unsigned long long: 8
Size of float: 4
Size of double: 8
Size of long double: 12
Size of void*: 4

Test new and delete
size: 48 pchar: 00991b54 psize: 00991b50 *psize: 48
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Test malloc and free
size of class __T: 8
size: 32 pchar: 00991b50 psize: 00991b4c *psize: 134253675

Test new and free
size: 16 pchar: 00991b54 psize: 00991b50 *psize: 16
~~~~~~~~~~~~~~~~

Test malloc and delete
size of class __T: 8
size: 8 pchar: 00991b54 psize: 00991b50 *psize: 8
~~~~~~~~